Energy Metering for Free: Augmenting Switching Regulators for Real-Time Monitoring

We present iCount, a new energy meter design. For many systems that have a built-in switching regulator, adding a single wire between the regulator and the microcontroller enables real-time energy metering. iCount measures energy usage by counting the switching cycles of the regulator. We show that the relationship between load current and switching frequency is quite linear and demonstrate that this simple design can be applied to a variety of regulators. Our particular implementation exhibits a maximum error of less than plusmn20% over five decades of current draw, a resolution exceeding 1 muJ, a read latency of 15 mus, and a power overhead that ranges from 1% when the node is in standby to 0.01 % when the node is active, for a typical workload. The basic iCount design requires only a pulse frequency modulated switching regulator and a microcontroller with an externally-clocked counter.